Flip relation between parent and sub modules and clarify terminology

[haqadn]

Proposed changes:

• Reference modules from sub modules instead of doing it the opposit way.
• Cleanup terminology and module code

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Make sure all module/submodule functionality is working as expected. Including:
- All modules and sub modules are changing front-end as expected
- Cache is being clearerd on any front-end change
- Cron jobs and scheduled jobs are registering as expected
- Cron jobs and scheduled jobs are being handled as expected

[github-actions]

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